The invention relates to mirrors and to a method of producing mirrors.
European patent specification EP 0 583 871 A1 describes a method of producing mirrors by applying a coating comprising a reflecting layer and at least two reflection enhancing layers to a ribbon of hot glass, for example a ribbon of float glass, during the glass production process. The layer furthest from the source of light to be reflected in use is regarded as the reflecting layer, with the reflection enhancing layers being between the light source and the reflection layer.
The reflecting layer may be a layer of high refractive index, for example silicon, a silicon oxide having a refractive index of at least 1.9, tantalum oxide, tin oxide or titanium oxide, with the reflection enhancing layers being alternatively of relatively low refractive index and high refractive index. A reflection enhancing layer of high refractive index may be, for example, of any of the high refractive index materials listed above for the reflecting layer. However, since silicon has a higher absorption for visible light than the metal oxides listed, it is generally preferred, according to the teaching of EP 0 583 871 A1, to use, for a back surface mirror, a metal oxide layer as the high refractive index inner reflection enhancing layer in order to achieve the required high visible light reflection. Thus, in a preferred structure for a back surface mirror, the arrangement of layers is
glass PA1 inner (reflection enhancing) layer of metal oxide PA1 intermediate (reflection enhancing) layer of relatively low refractive index PA1 outer (reflecting) layer of silicon.
The reflection enhancing layer of low refractive index may be of silicon oxide having a refractive index lower than that of the reflecting layer and that of the high refractive index reflection enhancing layer and in any event less than two.
At least the reflection enhancing layers are formed of layers of very approximately n.lambda./4 thickness, wherein n is an odd integer (preferably 1) and .lambda. is a wavelength of light in the visible region of the spectrum, such that the layers act to enhance the reflection by an interference effect.
Mirrors in accordance with the invention of EP 0 583 871A1 have a number of important advantages over conventional mirrors. Not only can they be produced by applying the reflecting coating to the glass during the glass production process (so avoiding the need for a separate off-line process to apply the reflecting coating), but the coating layers may be applied by pyrolytic processes (e.g. chemical vapour deposition) utilising the heat of the glass to produce pyrolytic coatings of high durability. However, there is a tendency for the mirrors to exhibit a reflection colour which is significantly more green than a conventional mirror. This tendency to a green reflection generally increases with increased reflection (as the thickness of the layers, especially the reflection enhancing layers, more closely approximates to n.lambda./4, where .lambda. is the wavelength of light towards the middle of the visible region of the spectrum). Moreover, it is especially significant in back surface mirrors where most of the reflected light passes twice through the thickness of the glass which may impart a green tint to transmitted light as a result of the presence of ferrous iron in the glass.